(1) Field of the Invention
This invention relates generally to the field of distance measurement and relates more specifically to distance measurement using capacitive sensors.
(2) Description of the Prior Art
Capacitive sensors are capacitors separated by a dielectric material, often the electrodes used have the form of plates. Using the properties and knowledge of electric field and capacitance between separated charged electrodes/plates it is possible to retrieve information about distance, pressure, etc. The distance between the plates or the active area of neighboring plates, i.e. the capacitance can be used to measure a distance of objects.
The capacitance C is a measure of the charge stored on each plate for a given voltage where
      C    =                  ɛ        ×        A            d        ,wherein ∈ is the permittivity of the dielectric, A is the area of the plates and d is the distance as the plates are separated. As the plates separate there is a direct relation between the capacitance and the distance d of the plates. In case the plates shift in parallel there is also a direct relation between the distance of the shift of the plates and the capacitance. This relationship can be used in sensing position and proximity by using the capacitance relationship to displacement.
It is a challenge for the designers of capacitive distance measurement systems to achieve a high accuracy, a long term stability and a low-cost design.
There are patents or patent publications dealing capacitive distance measurement systems:
U.S. Patent Publication (US 2005/0088184 to Burdick et al.) teaches a capacitive sensor that includes a variable capacitor transducer that varies its capacitance with changes in an environmental parameter. The present invention is adapted to measure any linear parameter such as pressure, force, or distance. The sensor of the present invention is compact, inexpensive to make, and easily fabricated using commonly available components. Furthermore, it is not susceptible to errors caused by vibration, acceleration, and its orientation to the earth's gravitational field. The output of the capacitive sensor does not substantially drift with changes in temperature.
U.S. Patent (U.S. Pat. No. 7,256,590 to Kitaoka) discloses a measuring apparatus having a probe that faces a surface of a target and is configured to supply AC current to the surface, measuring a voltage drop through a space between the probe and the surface, and obtaining a distance between the probe and the surface in accordance with the measured voltage drop. The apparatus includes a ground member facing, and apart from, the surface and configured to ground the surface by capacitive coupling, and a stage configured to hold either of the target and the probe and to move to define a measurement area on the surface. The ground member is configured so that the ground member faces all areas of the surface with respect to each of a plurality of measurement areas on the surface defined by a position of the stage.
U.S. Patent (U.S. Pat. No. 6,486,681 to Weber et al.) discloses a measuring circuit for a capacitive sensor for distance measurement and/or space monitoring comprising sensor wire and shielding electrode, a sine signal is applied to the shielding electrode. The sensor wire is connected, via a shielded cable, with one input of an input amplifier which serves as current-voltage converter and whose supply voltage is likewise influenced by the sine signal. The output of the input amplifier is connected with one input of a phase-dependent rectifier arrangement, the sine signal is applied to the other input of the phase-dependent rectifier arrangement, and its output is connected to an analog-to-digital converter.